1. Field of the Invention
The present invention relates to implantable medical devices. More specifically, the present invention relates to implantable medical devices that provide diagnosis and/or therapy for sleep disordered breathing.
2. Description of the Related Art
Sleep-disordered breathing (SDB) encompasses a variety of conditions including, for example, central sleep apnea and obstructive sleep apnea. In both instances, a cessation of breathing (apnea) and/or shallow or slow breathing (hypopnea), occurs on an intermittent or periodic basis. The apnea is generally terminated by an arousal from sleep leading to a disturbance of the sleep pattern. Such interrupted sleep often causes excessive daytime sleepiness, depression, irritability, memory impairment, and headaches. In addition, sleep-disordered breathing can be life threatening and an increasing correlation between sleep-disordered breathing and hypertension, diabetes, stroke, arrhythmia, heart failure and heart attacks is being established.
Obstructive sleep apnea is the result of a blockage of a portion of the upper airway, usually associated with a relaxation in muscle tone and/or a reduction in the size of the airway due to, for example, excessive fatty tissue. This mechanical blockage creates a pressure differential that further facilitates the apnea.
Central sleep apnea is a neurological disorder, wherein normal breathing patterns are interrupted due to a failure of the brain to generate the proper muscle stimulation pulses. Once initiated, the resultant apnea is eventually terminated with a resumption of respiration. Central sleep apnea can precede obstructive sleep apnea and this combination is referred to as mixed sleep apnea.
One particular variant of central sleep apnea that is often associated with patients suffering from chronic heart failure is Cheyne-Stokes respiration. Cheyne-Stokes respiration is a pattern of breathing characterized by a waxing and waning of tidal volume with complete cessation of breathing. Typically, a cycle of Cheyne-Stokes respiration lasts about 30–90 seconds. The cycle then repeats itself.
There are a variety of treatment options available for addressing sleep-disordered breathing. The most common treatment is the use of CPAP (Continuous Positive Airway Pressure). The patient generally wears an appliance such as a full facemask or more typically a nose covering mask or nasal inserts that deliver pressurized air into the airway to maintain the airway in an open state. The therapy is effectively a pneumatic stent. CPAP is generally effective at treating both central and obstructive apnea; however, many patients do not tolerate the therapy and discontinue its use.
Another method used to address sleep-disordered breathing involves atrial overdrive pacing (AOP). That is, an implantable medical device (IMD) is implanted to pace the heart. The pacing rate is elevated from a normal resting or sleeping rate (e.g., normal nocturnal intrinsic rate or a normal paced sleeping rate). For example, such techniques are described in U.S. Pat. No. 6,126,611, assigned to Medtronic, Inc., which is herein incorporated by reference in its entirety.
The use of AOP to address sleep-disordered breathing and its exact mechanisms are currently being investigated. The elevated pacing rate may lead to arousal such that apnea is terminated; however, it is believed more likely that the maintenance of heart rate and the increase in cardiac output achieved via AOP may positively affect autonomic tone sufficiently to reduce the overall number of apneas without causing arousals. While promising, the use of AOP does not always provide a successful therapy for sleep-disordered breathing.
There are various other therapies that can be delivered by an IMD to interrupt, terminate, or prevent episodes. Such therapies include, for example, variations in cardiac pacing, neural stimulation, muscle stimulation, patient alerts, and working in conjunction with an external device, such as the CPAP machine to tailor therapy.
In delivering such therapies, the IMD typically responds to a determination that sleep disordered breathing is present. In a broad sense, this could include simply delivering a given therapy whenever the patient is asleep (detected) or presumed asleep (night time hours). In that case, the patient would be evaluated in a lab and found to have sleep disordered breathing. The IMD would then be appropriately programmed.
More likely, the IMD would include one or more sensors that identify indicators of sleep disordered breathing. The sensors could be directly coupled with the device or communicate remotely via telemetry For example, impedance sensors could be used to monitor minute ventilation for breathing patterns. Similarly, air flow or temperature sensors could also monitor breathing. Heart rate, blood oxygen levels, or carbon dioxide levels are other indicators that can be monitored and correlated to sleep disordered breathing events.
Thus, the IMD gathers data to identify the onset of sleep disordered breathing and then responds. Alternatively, the data is used to pattern and/or predict the onset of SDB and provide the therapy accordingly. Some difficulty exists in accurately identifying SDB events. For example, variations in heart rate during sleep may indicate SDB events whereas the same variations during the day may result from intermittent exertion. Thus, there is continued need to further improve the accuracy of sleep disordered breathing event detection.